In a cottage high atop Llanfair Mountain, sixteen-year-old Clara lives with her sister, Maren, and guardian Auntie. By day, they gather herbs for Auntie’s healing potions; by night, Auntie spins tales of faraway lands and wicked fairies. Clara’s favorite story tells of three orphaned infants—Clara, who was brought to Auntie by a stork; Maren, who arrived in a seashell; and their best friend, O’Neill, who was found beneath an apple tree.

One day, Clara discovers iridescent scales just beneath her sister’s skin: Maren is becoming a mermaid and must be taken to the sea or she will die. So Clara, O’Neill, and the mermaid-girl set out for the shore. But the trio encounters trouble around every bend. Ensnared by an evil troupe of traveling performers, Clara and O’Neill must find a way to save themselves and the ever-weakening Maren.

And always in the back of her mind, Clara wonders, if my sister is a mermaid, then what am I?